Identifiability of sorption and diffusion processes using EIS: Application to the hydrogen reaction

In this work, a test is presented to determine the theoretical identifiability of electrochemical models involving adsorption and absorption of intermediate species coupled to diffusion. Analysis of theoretical identifiability is needed before the practical identification in order to know whether the model parameters may be identified globally (unique solution) or locally (a finite number of solutions), or if they are non-identifiable (infinite number of solutions). An experimental identification procedure of the system parameters is also proposed, this method being adequate even for theoretically non-identifiable models. The identifiability test is applied to the analysis of hydrogen evolution and insertion in metallic substrates. To illustrate the proposed procedure, electrochemical impedance spectroscopy (EIS) data were used for the experimental estimation of the hydrogen reaction parameters on steel electrodes.